Prices vary depending on the complexity of the presentation, the overall length of the content and the number of viewers that you are expecting.
Your webcasting provider should be able to give you a quote for a specific production very quickly but only if you can provide them with a good description of the scope of the project. The more online examples a provider can show you (from existing clients), the easier it is to say "how much is one of those?"
If you already have video content then prices fall drastically but you should also consider repurposing the existing footage to be more effective on the web. A short introduction and conclusion for an existing piece of video with a web specific call to action can be filmed very economically.
A CEO style message introducing the company or a new product or service is also a cost effective way of engaging your website visitors. Those of us old enough to remember Victor Kiam ("I liked [the shaver] so much I brought the company") are grateful that this sort of direct sell has become much subtler in recent years. But a simple, honest and personal message is actually much more powerful on the web than on any other medium.
We use the phrase "your best sales pitch every time" and it's true...webcasting enables you to deliver a powerful, consistent message time after time to thousands of potential customers.
At the other end of the scale, an all singing, all dancing production (sometimes literally) will obviously cost more. However, while quality should not be diminished just because the final version is for internet delivery, simplicity is the key to a successful web campaign and that means the production should cost a fraction of what you'd expect to pay for a full scale TV ad.
Streaming costs are determined by the number of viewers to a particular video and prices vary dramatically. This is often because a client cannot give an accurate indication of the number of viewers they are expecting. The more specific you can be ("it's going on the homepage which currently gets 20,000 unique impressions a month"), the more cost effective the quote. That may sound odd but we buy wholesale bandwidth depending on the number of live projects we have on the go. Being able to predict the requirement accurately makes our buying more efficient and we can pass that saving on to you.
Always decide on the criteria for what you consider to be an acceptable ROI before you embark on a webcasting project - an experienced streaming provider should be able to help you with this. One of our customers - an established online retailer - was surprised when their sales doubled from the day they added video demonstrations of their products to their website. We weren't surprised at all and by analysing the live stats coming back from the twenty or so different videos, we were also able to tell them which were the best sellers.